ReferenceError: "x" is not defined (V8-based & Firefox)ReferenceError: Can't find variable: x (Safari)

ReferenceError.

There is a non-existent variable referenced somewhere. This variable needs to bedeclared, or you need to make sure it is available in your current script orscope.

foo.substring(1); // ReferenceError: foo is not defined

The "foo" variable isn't defined anywhere. It needs to be some string, so that theString.prototype.substring() method will work.

const foo = "bar";foo.substring(1); // "ar"

A variable needs to be available in the current context of execution. Variables definedinside afunction cannot beaccessed from anywhere outside the function, because the variable is defined only in thescope of the function

function numbers() {  const num1 = 2;  const num2 = 3;  return num1 + num2;}console.log(num1); // ReferenceError num1 is not defined.

However, a function can access all variables and functions defined inside the scope inwhich it is defined. In other words, a function defined in the global scope can accessall variables defined in the global scope.

const num1 = 2;const num2 = 3;function numbers() {  return num1 + num2;}console.log(numbers()); // 5

• Scope
• Declaring variables in the JavaScript Guide
• Function scope in the JavaScript Guide